Preventing Text File Destruction in Shell – Common Mistakes

io-redirectionshelltext processing

I have destroyed a bunch of non-essential files and I don't know why. I have been executing commands like:

tr -sc 'A-Za-z' '\n' > somefile.txt | less

there is no output (blank page with flashing END) and upon checking all the content from the file is erased.

Another command that erased a full text file

grep someword > someotherfile.txt  | less

Best Answer

The > operator means "take the output of the command, truncate the named file, and then write the output of the command to that.

Reading that command line I guess you want <, which is "read standard input from this file, and feed it to the command" instead.

Related Solutions

Bash – Using text from previous commands’ output

You could use script (use -f to make sure it flushes every time) and you can have all the output in a file.

Use a terminal multiplexer (screen, tmux etc). For instance, for screen http://web.mit.edu/gnu/doc/html/screen_11.html#

Command: copy Enter copy/scrollback mode. This allows you to copy text from the current window and its history into the paste buffer. In this mode a vi-like full screen editor is active, with controls as outlined below.

Other terminal multiplexers surely have more advanced capabilities.

Text Processing with Sed – Deleting Commented Lines in a Text File

Use the sed delete command (here assuming GNU sed as found on Ubuntu and other GNU systems).

 sed -i '/^[@#]/ d' sample.txt

If you need to account for leading space characters:

sed -i '/^\s*[@#]/ d' sample.txt

Best Answer

Related Solutions

Bash – Using text from previous commands’ output

Text Processing with Sed – Deleting Commented Lines in a Text File

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